In the future, human consciousness can be stored on a "stack", a memory device that records your memories and personality and everything else that defines "you". The stack is implanted at the base of the head. When you perish, your stack can be down loaded into a new body. Of course, the stack can be backed up, placed in storage, or (as is the case for our hero, Kovacs) transmitted as a stream of digitized information from one planet to the next. (Coincidentally, this description of stored consciousness being restored to a body sounds similar to Cory Doctorow's Down and Out in the Magic Kingdom, which I read not too long before this one. But both books are radically different in tone and story. Morgan also explores the idea of downloading the human consciousness into a body not your own.)
Kovacs is an ex-envoy, a specially trained, enhanced killing machine. He is augmented with capabilities for great speed and lethal damage. Unfortunately, he has been caught for crimes and his consciousness is sentenced to a long period of down time. Someone on planet Earth hires Kovacs, so his identity is transmitted across the cosmos and downloaded into a new body, or "sleeve". A very wealthy, very long lived (he keeps fresh clones of his body in storage, so he can continue to download into younger, healthier versions of himself) man named Bancroft wants Kovacs to investigate why he (Bancroft) apparently committed suicide. Bancroft is convinced he is not the type to kill himself, so he concludes he must have been murdered even though the police say it was clearly suicide. Unable to get the police to investigate further, Bancroft has hired Kovacs, with his special envoy skills to investigate the mystery. (After the suicide, Bancroft is restored from the last backup of his consciousness, which was made a few days before his death, and thus Bancroft has no recollection of what happened.)
The action takes place on 25th century earth, in the San Francisco area (the Golden Gate bridge still stands, but it is a rusty piece of junk.) Because bodies are disposable, they are treated badly - why not smoke or enjoy high risk entertainment if you know you can just download into a new healthy body if something bad happens? It seems there are people who don't want Kovacs to investigate, and violence results. Lots of nice touches, the story is pretty good and the action keeps it moving forward.